Alterations in gait biomechanics are common during early stages of Parkinson’s disease (PD), potentially elevating energy requirements of walking and leading to impaired economy of gait. Although gait economy is traditionally assessed during treadmill walking with simultaneous ox-ygen consumption (VO2) monitoring, treadmill gait mechanics, particularly in PD, may be different from over-ground walking mechanics, possibly providing a distorted picture of true gait economy. Currently, no studies have directly examined the energy cost of over-ground walking in PD patients. The purpose of this study was to test the feasibility of measuring energy expenditure during over-ground walking in mild to moderate PD using portable gas exchange monitoring technology. Additionally, we sought to determine whether energy expenditure, as assessed through VO2 measures, related to disease severity for PD. Seventeen PD patients underwent separate 6-minute walk (6MW) tests both with and without the COSMED K4b2 portable oxygen monitoring system. Gait economy was calculated as measured VO2 during 6MW divided by the predicted VO2 for non-PD age-matched subjects, according to a standard estimation equation utilizing ground speed. Distance covered during the 6MW with the portable system (420 ± 12 meters) was highly correlated (r = 0.96, p < 0.001) to distance without the system (442 ± 135 meters) indicating that the lightweight equipment did not confound walking ability in these participants. Mean VO2 during the 6MW (16.0 ± 5.5 mL/kg/min) was 74% of mean VO2 peak for this sample, and 16 out of 17 subjects had impaired gait economy. The degree of gait economy impairment was not related to scores on either UPDRS total or motor. Our results demonstrate that PD patients use extraordinarily high percentage of VO2 peak for normal floor walking, and show impaired gait economy relative to prediction equations. Interestingly, the degree of elevated energy expenditure during gait did not relate to disease severity.